The field of heat exchanger designs is replete with applications of fins to improve the heat transfer. Typically this is heat transfer by forced convection mechanism. The heat transfer by forced convection takes place between a solid surface and fluid in motion, which may be gas or liquid, and it comprises the combined effects of conduction and fluid flow. This type of heat transfer occurs in most of the conventional heating systems, either hot water or electric and industrial heat exchangers.
In the cracking process of a paraffin such as ethane or naphtha, the feed flows through a furnace coil (pipe) that is heated up to 1050° C., inside the radiant section of a cracking furnace. At these temperatures, the feed undergoes a number of reactions, including a free radical decomposition (cracking), reformation of a new unsaturated product and the coproduction of hydrogen. These reactions occur over a very short period of time that corresponds to the feed residence time in a coil.
The interior of the radiant section of the furnace, is lined with heat absorbing/radiating refractory, and is heated typically by gas fired burners. The heat transfer within the furnace, between flame, combustion gases, refractory and the process coils is mostly by radiation, and also by forced convection.
There is a drive to improve the efficiency of cracking furnaces as this reduces process costs and green house gas emissions. There have been two main approaches to improving efficiency; improving heat transfer to the furnace coils, i.e. from flame, combustion gases and refractory walls to the external surface of a process coil, and improving heat transfer within the coil, i.e. from the coil walls into the feed flowing inside the coil.
One of the methods representing the second approach, is the addition of internal fins to the inner walls of the furnace coil, to promote the “swirling” or mixing of the feed within the coil. This improves the convective heat transfer from the coil walls to the feed as the turbulence of the feed flow is increased and the heat transferring surface of the hot inner wall of the coil is increased as well.
U.S. Pat. No. 5,950,718 issued Sep. 14, 1999 to Sugitani et al., assigned to Kubota Corporation provides one example of this type of technology.
The papers “Three dimensional coupled simulation of furnaces and reactor tubes for the thermal cracking of hydrocarbons”, by T. Detemmerman, G. F. Froment, (Universiteit Gent, Krijgslaan 281, b9000 Gent—Belgium, mars-avri, 1998); and “Three dimensional simulation of high internally finned cracking coils for olefins production severity”, by Jjo de Saegher, T. Detemmerman, G. F. Froment, (Universiteit Gent1, Laboratorium voor Petrochemische Techniek, Krijgslaan 281, b-9000 Gent, Belgium, 1998 provide a theoretical simulation of a cracking process in a coil which is internally finned with helicoidal and longitudinal fins (or rather ridges or bumps). The simulation results are verified by lab scale experiments, where hot air flows through such internally finned tubes. The papers conclude that the tube with internal helicoidal fins performs better then with internal longitudinal fins and that the results for “a tube with internal helicoidal fins are in excellent agreement with industrial observations”. However, no experimental data are provided to support these conclusions. There is also no comparison made to the performance of a bare tube, with no internal ribs or fins. The authors agree that one potential disadvantage of such coils with internal fins is that carbon deposits may build up on the fins, increasing the pressure drop through the tube.
The addition of fins to a coil external surface are taught as well in the prior art. U.S. Pat. No. 5,590,711 issued Jan. 7, 1997 to Ishida et al., assigned to Kabushiki Kasha Kobe Seiko Sho, discloses heat exchanger tubes having a plurality of external crests and ridges on their surface. The tubes are used in refrigeration and air conditioning applications, in which a liquid (e.g. water) is in direct contact with the external surface of the tube. The patent does not suggest the tubes could be used in a radiant section of a cracking furnace.
U.S. Pat. No. 7,096,931 issued Aug. 29, 2006 to Chang et al., assigned to ExxonMobil Research and Engineering Company teaches an externally finned heat exchanger tube in a slurry reaction (Fischer Tropsch synthesis). In the reaction, a slurry of CO and hydrogen in a hydrocarbyl diluent containing catalyst, flows over heat exchanger. tubes containing flowing cooling water. The water is heated to steam in the process, to remove the heat of reaction.
Both of the preceding patents teach heat exchange by forced convection. That is a flowing fluid (water or, a hydrocarbon) is in contact with the external surface of a heating/cooling tube which has groves or fins on its surface. Neither of the patents suggests external protuberances to enhance the radiative heat transfer to the tube.
NOVA Chemicals U.S. Pat. No. 7,128,139 issued Oct. 31, 2006 teaches external annular fins on the cracking furnace coil to increase convection heat exchange to the coil.
Canadian Patent application 2,746,285 titled Furnace Coil Fins filed Jun. 29, 2011, in the name of NOVA Chemicals discloses longitudinal fins on the passes of a furnace tube. This does not suggest protuberances of the present invention.
U.S. Pat. No. 4,402,359 issued Sep. 6, 1983 to Carnavos et al., assigned to Noranda Mines Limited, discloses a heat transfer device having an augmented wall surface that comprises a base wall of heat conductive material having plurality of pyramid fins, regularly spaced apart and formed as a knurled diamond pattern around the outside periphery of the pipe. However, the fins are of a very small size of 0.015-0.040 inch, and their primary purpose was to enhance the convective heat transfer from a flowing liquid into a solid surface.
U.S. Pat. No. 4,232,728 issued Nov. 11, 1980 to Fenner et al., assigned to Union Carbide discloses a metal tube for a heat exchanger, with enhanced heat transfer capabilities that has an inner surface provided with a layer of randomly distributed irregular metal bodies or particles bonded to the inner surface and spaced from each other. The ratio of the arithmetic average height of these bodies or particles to the tube inner diameter is at least 0.006.
U.S. Pat. No. 7,386,980 issued Jun. 17, 2008 to Green et al., assigned to Power Systems Mfg LLC discloses a combustion liner having enhanced heat transfer, which has substantially uniform pattern comprising a plurality of intersecting grooves textured into its cooling surface. The texture can be also shapes as e.g. frustums of pyramids, with heights 0.01 inches.
All the above references teach away from the subject matter of the present invention as the surface modifications are geometrically very small so they represent rather grooves or material “texture”, and they were designed to increased convective heat transfer between fluids, primarily in heat exchangers.
The present invention seeks to provide a method to increase the net radiant and convective heat capture by a process coil in the radiant heating section of a cracking furnace.